This is a proposal for the development and clinical application of adaptive phased array ultrasonic imaging systems and the continued investigation of compound ultrasonic imaging. Over the past grant period, extensive test tank and clinical studies have defined many of the physical principles affecting the viability of these imaging modes. Based on the results, the design and construction of a real-time adaptive phasing imaging system capable of generating diffraction-limited resolution images through aberrating media such as abdominal and breast fat and skull is proposed. It is hypothesized that such a system will markedly improve ultrasonic image quality in a wide range of clinical applications and it is proposed to undertake comprehensive studies of the origin of phase errors, their effects on image quality, and methods for eliminating these effects. These goals represent a change from those of the current grant period and are a result of the discovery of and early positive results achieved with a new technique for adaptive imaging. Clinical studies involving abdominal and breast imaging will assess adaptive imaging techniques for those applications for both one- and two-dimensional arrays. Direct time of flight measurements across human breasts and abdominal wall are also proposed in order to define the magnitude and spatial characteristics of phase aberrations in vivo. Further, it is proposed that the four-to-one array multiplexing system constructed and evaluated during the previous grant cycle be expanded to a seven-to-one system. This system will be used to extend the compound imaging studies and will provide more than twice the speckle reduction and specular target boundary definition that the four-to-one system. Also, preliminary experience with in vivo phase measurements and phase correlation schemes indicate that the formation of very high resolution synthetic aperture images is possible with this system. Such images are formed by summing the radio frequency signal from sub-apertures, rather than the envelope-detected signal summed in compound imaging. Previously, synthetic aperture imaging was limited by phase errors introduced by tissue and transducer motion during signal acquisition. We propose to utilize the newly developed adaptive phasing schemes to overcome these limitations and conduct off-line and, ultimately real time clinical studies to investigate ultrasonic synthetic aperture imaging.